Posted by Jesse Brennan on December 11, 1999 at 14:21:23:
In Reply to: Re: steam defects-drying out plaster mold posted by bruce paul fink on December 10, 1999 at 08:29:50:
Gypsum plaster contains water in two forms , water adsorbed into the material and water that is reacted with the plaster.
The adsorbed water is best removed by heating to above 220 F thru the whole mass of the mold. In practice the mold is at 400 F to 500 F for a period long enough to get the 220 F minimum temperature though out the mold . Some molds in industrial practic will be quite large and be baked at the 500 F temperature for 12 to 16 hrs. This is sufficient bake out if Aluminum or Zinc based alloys are to be cast. In this case molds are removed from the oven at 120 F to 150 F and poured immediately. if the calcined mold is allowed to sit it will readsorb moisture from the air and the pour will likely fail.
For Copper based alloys The bake out temperature is extended to produce a temperature of 1200 F through out the mold idealy and care must be taken not to overheat it because the plaster is at the decomposition point at this temperature. Again the mold must be poured immediately.
Bruces derived 900 F final temperature gives a resonable balance between retained mold strength and moisture (steam elimination).
A very permeabile mold that still has a good surface is most desired. The plaster water ratio needs rto be carefully controlled 1-1 water plaster ratios are common in industrial practice. Air entrainment will also help in making the mold permeabile. Air frothing to increase the volume by 70% to !00% is not uncommon in industrial practice. The air addition has to be done properly to have a bubble free casrting surface. I haven't tried air frothing. US gypsum makes a special high permeabile plaster called Hydro Perm for the high Air entrainment but it is not readily available. If you are near Detroit you can probably find it. In Texas no way.
I'm not an expert on this but I do know how to search stuff out.